Wednesday, 4 February 2009

Shedding leaves

Tomorrow I'll talk to a journalist from AAP about the Breen Sculpture Competition (to design a sculpture feature for our first children's garden at Mount Tomah Botanic Garden - have a look at our website http://www.rbgsyd.nsw.gov.au/, top right corner).

That's exciting enough in itself but we'll also talk about deciduous trees! Not important in Australia you might say. Well of course we have lots of exotic deciduous trees in our gardens. We also have a few deciduous natives.

The most well known, and most aptly named, is Deciduous Beech (Nothofagus gunnii) from Tasmania. The White Cedar (Melia azederach), growing north of Sydney, is another that loses its leaves in winter.

More commonly in Australia, we have plants that lose their leaves in the dry season or in drought. There are plenty of tropical examples, including eucalypts. The Deciduous Figs (Ficus virens) in the Domain usually drop all their leaves for a few weeks in October or November.

And why do trees drop their leaves all in one hit? Presumably to conserve water and better survive tough conditions by relieving themselves of fragile leaves when they are not actively growing. In bitterly cold northern hemisphere winters its better to not have your leaves hanging around. In tough Australian dry seasons why keep your leaves.

Sometimes its helpful to shed some or all leaves so that your flowers are easier for birds and bugs to find, and pollinate. The Illawara Flame Tree (Brachychiton acerifolius) and the Red Cedar (Toona ciliata) are two local examples.

Barbara Wiecek (who with Karla Davies checked this information for me) told me that during droughts her local northern hemisphere deciduous trees get their 'autumn colour' and drop leaves in the middle of summer, and then again in winter.

5 comments:

Since moving to the tropics, I too have become curious about deciduous trees here. The Northern Hemisphere conceit that leaves are lost because of cold and snow simply does not apply. I found a classic article by D.H. Janzen that covered this fascinating subject - and more. Trees drop their leaves for a multitude of reason. Here's the complete reference:

Thanks Mary. I'll try and chase that article up. A lot of biology still has this 'quaint' historical bias - e.g. it's very hard to shake the taxonomic categories established by Europeans for organisms that occur all around the world but were first studied from a small subset in England!

Seanna McCune from our Botanical Information Service provides the following advice:

"Deciduous trees might be considered cleaner and tidier from a leaf shedding point of view, as they shed all their foliage at once, usually during autumn or winter, and remain clean for the rest of the year. I suggest one of the deciduous magnolias or a Japanese Maple would be suitable deciduous tree near a swimming pool."